Amine Hole Scavengers Facilitate Both Electron and Hole Transfer in a Nanocrystal/Molecular Hybrid Photocatalyst

A well-known catalyst, fac-Re(4,4′-R2-bpy)(CO)3Cl (bpy = bipyridine; R = COOH) (ReC0A), has been widely studied for CO2 reduction; however, its photocatalytic performance is limited due to its narrow absorption range. Quantum dots (QDs) are efficient light harvesters that offer several advantages, including size tunability and broad absorption in the solar spectrum. Therefore, photoinduced CO2 reduction over a broad range of the solar spectrum could be enabled by ReC0A catalysts heterogenized on QDs. Here, we investigate interfacial electron transfer from Cd3P2 QDs to ReC0A complexes covalently bound on the QD surface, induced by photoexcitation of the QD. We explore the effect of triethylamine, a sacrificial hole scavenger incorporated to replenish the QD with electrons. Through combined transient absorption spectroscopic and computational studies, we demonstrate that electron transfer from Cd3P2 to ReC0A can be enhanced by a factor of ∼4 upon addition of triethylamine. We hypothesize that the rate enhancement is a result of triethylamine possibly altering the energetics of the Cd3P2–ReC0A system by interacting with the quantum dot surface, deprotonation of the quantum dot, and preferential solvation, resulting in a shift of the conduction band edge to more negative potentials. We also observe the rate enhancement in other QD–electron acceptor systems. Our findings provide mechanistic insights into hole scavenger–quantum dot interactions and how they may influence photoinduced interfacial electron transfer processes.

Sample Preparation. Previous methods of adsorbing molecules to QD surfaces involve adding an excess of the molecule and sonicating in a solvent (heptane) in which the quantum dot is soluble, but not the molecule. Instead, the samples were freshly prepared on the day experiments were performed by adding ReC0A in MeCN (2 mL) to a volume of Cd3P2 in chloroform (~0.33 mL, 75 μM) corresponding to a UV-Vis absorbance of 0.3 optical density (OD) for both in a 1:1 ratio of ReC0A (400 nm) to Cd3P2 (exciton, ~700 nm). The sample was rotary evaporated, dissolved in 1 mL of heptane to result in a final QD concentration of 25 μM, and sonicated for 3 hours to allow the ReC0A to bind to the QD. We added ReC0A to the QD solution and determined through FTIR spectral analysis that approximately 60% of the ReC0A added bound to the surface, resulting in a final concentration of 0.52 mM and an average of 21 Re complexes per QD. All samples were transferred into a 1 mm optical glass cuvette obtained from Starna and stirred for experiments. For experiments with the hole scavenger, 10% TEA was added to the already prepared Cd3P2-ReC0A samples in the dark and were wrapped in aluminum foil when transporting the cuvette. All chemicals were used as purchased and the ReC0A was synthesized using the following procedure by the Kubiak group at University of California San Diego. The TA experiments were compared with and without Ar purging and no difference was evident.
Visible Femtosecond Transient Absorption Spectroscopy. A Coherent Systems Ti:Sapphire regenerative amplifier system (800 nm fundamental, 2 W power output, 150 fs pulse width, 1 kHz repetition rate) with a Helios spectrometer (Ultrafast Systems LLC) was used for the visible picosecond experiments. This setup is further described in a related study by Wu et al. 1 The 800 nm fundamental laser output was split to generate the pump and probe beams. The fundamental beam was frequency doubled through a BBO crystal to get the 400 nm pump. The beam was focused at the sample and a chopper was used, at a rate of 500 Hz. The second half of the 800 nm beam was focused onto a sapphire window to generate a white light continuum (WLC) as the probe. To get a ∆A spectrum, the WLC was split to form reference and signal beams and were focused into fiber coupled multichannel spectrometer with a complementary metal-oxide semiconductor (CMOS) for detection.

Transient Femtosecond Infrared Absorption Experiments. A Coherent Systems Astrella
Ti:Sapphire regenerative amplifier system laser system was used (800 nm, 5 W power output, 35 fs pulse width, 1 kHz repetition rate) with infrared (IR) and visible optical parametric amplifiers (OPAs) and an Helios Fire transient absorption spectrometer (Ultrafast Systems LLC) to do transient infrared experiments. The 500 and 600 nm pump beams were generated from the visible OPA by sum frequency generation of the signal and by second harmonic generation of the signal through a BBO crystal, respectively. The pump beams were chopped at a rate of 500 Hz to obtain a ΔA spectrum and were also directed onto a motorized delay stage to get transient results. From the IR OPA, the signal and idler were mixed in an AgGaAs DFG crystal to form the 5000 nm IR probe. An iHR 320 Horiba spectrometer (component of Helios Fire setup) was used with a 50 grooves/mm grating. A nitrogen cooled 128 x 128 pixel mercury cadmium telluride (MCT) detector was used. Samples for the TRIR experiments were prepared in the same way as was done for visible transient experiments, described above.

Time Correlated Single Photon Counting (TCSPC).
A mode-locked Ti:Sapphire laser (Tsunami oscillator pumped by a Spectra Physics 10 W Millenia Pro) was used with a fundamental output beam of 800 nm (~100fs, 80 MHz). The 800 nm beam traveled through a pulse picker (Conoptics, USA) and was then frequency doubled to generate the 400 nm excitation beam that would pass through the 1 cm cuvette containing the sample (right angle geometry). For detection, a microchannel plate photomultiplier tube (Hamamatsu R3809U-51) was used, and the analysis done by a TCSPC board (Becker & Hickel SPC 600). Similar to the transient absorption experiments, samples were prepared the day experiments were performed. With the QD and hole scavengers, Cd3P2 in chloroform was diluted 100x from the original amount used for TAS experiments (so that it had a UV-Vis absorbance of 0.1 OD in a 1 cm quartz cuvette) and 10% TEA was added (by volume). Samples were transferred to 1 cm cuvettes.
Electrochemistry and Cyclic Voltammetry (CV). Solvents were obtained from Fisher Scientific. Acetonitrile was degassed with argon, dried over alumina, and dispensed by a custom-made solvent system. Reagents obtained from commercial sources; pentacarbonylchlororhenium, decamethylferrocene (Fc*), and 2,2'-bipyridine from Sigma Aldrich, 2,2'-bipyridine-4,4'dicarboxylic acid from Alfa Aesar, Tetrabutylammonium hexafluorophosphate (TBAPF6, Aldrich, 98%) was recrystallized twice from methanol and dried at 90°C overnight. Triethylamine (Sigma ≥ 99%) was distilled from CaH2 and stored over 3Å sieves in a N2 glovebox prior to use. Experiments were performed on a BASi Epsilon potentiostat. Experiments were run in 0.1 M TBAPF6 in 5 mL acetonitrile (MeCN) with 1 mM catalyst for control experiments. Experiments involving TEA were run in 0.1 M TBAPF6 in 4.5 mL MeCN and 0.5 mL TEA with 1mM catalyst. Decamethyferrocene (1 mM) was used as an internal standard for all scans. The reported potentials were converted to NHE using literature values. [2][3] A 20 mL scintillation vial with a custom cap was used for all CV experiments. A 3 mm diameter glassy carbon working electrode, Pt wire counter electrode, and Ag/AgCl reference electrode (separated from solution in a glass tube filled with 0.1 M TBAPF6 in MeCN and fitted with a CoralPor tip). The glassy carbon electrode was polished with 15, 3, and 1 micron diamond successively then thoroughly rinsed with methanol and dried before use. The platinum wire was flame-treated with a butane torch before use. Ar and "bone dry" CO2 were run through Drierite columns and then through a sealed vial of dry MeCN filled with 3Å sieves. An oven dried cannula was used to transfer the MeCN saturated solution from the vial to the electrochemical set up. Electrochemical set-ups were sparged for at least 10 minutes prior to electrochemical experiments. Gas flow was continued over the solution during experiments. Ohmic drop was correct for by using the iR-compensation tool of the potentiostat. The tool corrected for between 90-100% of the measured resistance. HR TEM experiments were conducted at Oakridge National Laboratory. Aberration-corrected high-angle annular dark-field (ADF) and bright-field (BF) image pairs were obtained on a Nion UltraSTEM U100 operated at 100kV. Samples were deposited on lacey carbon grids and baked at 80 o C overnight under high vacuum to minimize hydrocarbon contamination. The average Cd3P2 diameter was determined to be 3.14 ± 0.208 nm ( Figure S2A-C) and the Cd3P2-ReC0A had an average diameter of 3.33 ± 0.211 nm ( Figure S2 D-F). Though these sizes aren't conclusive, it is clear from the images that the ReC0A did not etch the QD.

S4. Attempting to Reverse Exciton Band Blue Shifting
To reverse the blue shifting of the exciton band upon catalyst adsorption, 20% oleic acid (by volume) was added to the Cd3P2-ReC0A samples. Unfortunately, these results were inconclusive due to significant degradation of the QD. This experiment was repeated at two elevated temperatures, 57 and 108 o C, for both Cd3P2 and Cd3P2-ReC0A samples. After addition of ReC0A, there is a 20 nm blue shift that occurs and we found that this effect was not reversible upon addition of varying concentrations of OA (1 to 20%) and that there was a continual blue shift of up to 15 nm for both the QD and QD-ReC0A samples. We attribute the blue shift upon catalyst adsorption to strong electronic coupling of the catalyst and a change in the QD surface dipole as Cd3P2 is strongly quantum confined and more sensitive to its surface environment. This is suggested by a continual blue shift with increasing amounts of Rec0A shown below and we will further investigate this at a later date.

S5. UV-Vis of ReC0A on CdS and CdSe
UV-Visible spectra for CdSe and CdSe-ReC0A in heptane and CdS and CdS-ReC0A in CHCl3 and heptane, respectively. Note that there is no blue shift of the 1S exciton band upon addition of the ReC0A to either CdSe or CdS that was observed with Cd3P2. We attribute this to Cd3P2 being more quantum confined than the former QDs and more susceptible to surface dipole effects.

S7. Transient IR Spectra of ReC0A on Cd 3 P 2
We performed transient IR (TRIR) experiments using 500 nm and 600 nm pump light, both outside of the ReC0A absorption range, and used ~ 2000 cm -1 centered broadband probe. The results show the ReC0A does indeed receive an electron from the Cd3P2, and at an ultrafast rate ( Figure S7). The reduced Re species peak appears at a frequency ~20 cm -1 lower than that of the original species' bleach and within a 300 fs delay time. Others have also reported an ultrafast electron transfer, but have not speculated on the cause. 5 Figure S8. Transient IR spectra of ReC0A on Cd3P2 pumped with 500 nm at very high pump fluence showing that the Fano resonance signal forms almost immediately withing 0-1 ps. The offset is due to the excited electron signal in the quantum dot. A) Cd3P2-0.25xRe shows only a positive feature at 2025 cm -1 corresponding to the Fano resonance signal. B) Cd3P2-1xRe where Fano resonance can also be observed at early time, however, at ~2005 cm -1 there is a small absorption corresponding to the growth of the reduced ReC0A species. At later time, as the FR decays, that absorption becomes more pronounce and the ground state blaech of the complex is also observed at 2029 cm -1 . C) Cd3P2-2xRe. Similar features are observed here as part B, however, the decay of the FR is faster due to more ReC0A on the QD surface and there is also a larger reduced ReC0A signal corresponding to more reduced complex. D) Latest time delay (800-1670 ps) spectra overlayed for each sample after scaling 1x and 2xRe early time amplitudes to 0.25xRe data. As more ReC0A is added to the QD surface, there is more reduced ReC0A that appears faster.

Figure S9. (A) and (B)
show TA spectra at 1000-1670 ps for samples of each ReC0A concentration without and with TEA, respectively, zoomed into the 520 nm peak corresponding to reduced ReC0A seen in Figures 6C and 7C. Upon addition of TEA and faster bleach recovery, the reduced ReC0A signal becomes more visible.

S8. Loss of Bleach Amplitude
The loss of bleach amplitude we observe is most likely due to defect states as a result of OA likely being replaced upon ReC0A adsorption and ultrafast ET. With regards to the ultrafast ET, we can compare the unnormalized bleach amplitudes for Cd3P2 and Cd3P2-ReC0A at three different pump wavelengths: 400, 560, and 700 nm. Pumping at higher energy (400 and 560 nm) shows a bleach amplitude loss of almost half, while at 700 nm excitation, the bleach amplitude is the essentially the same for both samples. Both 400 and 560 nm pumps are at higher energy than the QD CB edge, so ultrafast ET is more likely to occur, whereas, at 700 nm, the same energy as the QD exciton band, there is most likely direct excitation to the CB edge. These results along with our hypothesis that electron trapping due to ReC0A adsorption occurs likely both contribute to the loss in bleach amplitude. Figure S10. Unnormalized kinetics of Cd3P2 and Cd3P2-ReC0A at A) 400 nm, B) 560 nm, and C) 700 nm excitation. The higher energy pump wavelengths result in ultrafast electron transfer due to exciting much higher than the conduction band edge, while pumping at 700 nm likely results in direct band edge excitation. The top four panels of Figure S6 show the evolution of TA spectra over time for the various concentrations of ReC0A on the Cd3P2 in heptane. As more ReC0A is added, the band blue shifts. While a blue shift often occurs from degradation of the QD, our TEM data shows that degradation is not occurring, as the size is slightly larger with the ReC0A. The bottom four panels show the same exciton bands after addition of 10% TEA. They show an even further blue shift. In both sets of panels, the 1.5x and 2x ReC0A concentrations show a clear charge separated state emerging over time with a derivative shape, the positive feature approximately between 400 nm and 650 nm and a negative feature between 650 nm and 720 nm.

S10. TA Spectra of Cd 3 P 2 + ReC0A with different concentrations of TEA
We performed TA experiments of Cd3P2-0.5xReC0A with varying concentrations of TEA to observe the effect on ET. We found that as you increase TEA concentration, ET also increases, shown in the figure below.

S11. Time-Correlated Single Photon Counting
The absorption and fluorescence spectra were taken of the Cd3P2 ( Figure S7A). The fluorescence spectrum was used to determine a suitable wavelength for collection for time-correlated single photon counting experiments, which were conducted using 400 nm to excite and detected at 780 nm. Time-correlated single photon counting experiments were performed with various concentrations of TEA, showing that at less than or equal to 17.7 mM, the TEA slightly enhanced the fluorescence (Figure 7B), and at concentrations that were higher, the TEA behaved as a hole scavenger ( Figure S7C), quenching the photoluminescence.  Figure S14. A) NMR spectra of Cd3P2 in CDCl3 with varying concentrations of TEA B) CH3 Oleic acid peaks and C) CH2 Oleic acid peaks show broadening with addition of low concentrations of TEA and return to original peak width with higher concentrations.

A
Samples were prepared by rotary evaporating the Cd3P2, chloroform, and redissolving in CDCl3 obtained from Sigma Aldrich (99.96% D atom). The concentration of the QD was calculated to be ~ 25 µM and varying concentrations, from 259 µM to 1.1 mM, of TEA in CDCl3 were added to the sample for the titration. For 1 H NMR studies, a Bruker Ascend 600 MHz spectrometer was used (frequency: 600.18 MHz). It also included a prodigy cryoprobe that was cooled with liquid nitrogen. Figure S8 shows H NMR spectra of Cd3P2 in CDCl3 with various amounts of TEA added.
DOSY NMR was performed on the same instrument described above with a TEA concentration of 459 µM (second lowest concentration). A gradient was applied increasing from 2 to 95% with 25 points taken. These points were then fit to the following equation to obtain the diffusion coefficient: Eq. 1 where is the intensity, 0 is the intensity at zero gradient strength, is the gyromagnetic ratio, is the gradient strength, and are the delays between pulses, and D is the diffusion coefficient. 6 Figure S15. DOSY NMR spectra of a) Cd3P2 and b) Cd3P2 with TEA. Figure S16. DOSY NMR spectra of TEA in CDCl3 provides a diffusion coefficient of 3.62 μm 2 /s.

S13. Cyclic Voltammetry of ReC0A and TEA
CV experiments were conducted on Re(bpy)(CO)3Cl (ReCl) in MeCN as a control. The first reduction of ReCl remains the same (-1.16 vs NHE) regardless of TEA's presence in solution and there is no appearance of a peak between the first and second reduction of the complex.  To determine if the small peak in the ReC0A electrochemistry was from the reduction of TEAH + , TEA oxidation and reduction was studied electrochemically. In a solution with TEA and no catalyst, there were not visible reduction peaks when scanning between 0.4 V and -2.0 V vs NHE. When the solution is scanned past 0.5 V vs NHE, there is a large increase in current corresponding to the oxidation of TEA. In the same scan, the reduction of TEAH + is now present at -1.56 V vs NHE. In the presence of ReCl, the oxidation of TEA is unchanged but, when the oxidation is followed by scanning cathodically, the reduction of TEA shifts positively to -1.34 V vs NHE. When this experiment is run in the presence of ReC0A, the reduction of the TEAH + appears at -1.29 V vs NHE, similar to the small peak seen in the TEA plus ReC0A solution. From this, we conclude that the change in the first reduction of the ReC0A upon addition of TEA and the appearance of a new reduction peak, is due to the deprotonation of the carboxylic acid on the bpy ligand.

S14. Kinetics of QDs and Electron Acceptors with and Without TEA
To study if the increased electron transfer rate after adding TEA was generalizable, we tested multiple QD-electron acceptor systems with and without TEA, in multiple solvents. Figure  S10, shows that this is indeed the case. We studied CdSe in heptane with ReC0A bound, CdS in

S15. Ab initio Method Details
Gaussian 16 version A.03 7 and Vienna Ab initio Simulation Packages (VASP) version 5.4.1 8-11 were used to on all density functional theory (DFT) calculations. The Vaspkit 12 program was used to calculate the work function and Jmol 13 was used for visualization.
Gaussian was used for ReC0A geometry optimizations and frequency calculations, which were performed using the (U)B3LYP functional 14-16 with the 6-311+G(2df,p) basis sets 17-25 on all non-metallic atoms and the def2-TZVP effective core potential and basis set 26-27 on the Re atom. for geometry optimization and frequency calculations. Scale factor 0.965 was applied to all frequency calculation results reported in this article.
VASP was used for optimizations of Cd3P2 unit cell, Cd3P2 slab, Cd3P2 slab with capping agent, and Cd3P2 slab with capping agent & ReC0A. The electron-ion interactions were described by the Perdew-Burke-Ernerhof (PBE) exchange-correlation functional 28 and the projected augmented-wave (PAW) method 29-30 . The Gaussian smearing method with a smearing parameter σ = 0.1 eV was applied for all the calculations. The DFT-D3 method with the Becke-Jonson damping 31-32 was used to describe dispersion interactions. The plane wave basis set was cutoff at 400 eV and the energy convergence criterion was set to be 10 -6 eV per unit cell. A Monckhorst-Pack 33 (MP) type k-point grid of 9 × 9 × 9 was used for the optimization of Cd3P2 unit cell; an MP type k-point grid of 3 × 3 × 1 was used for the optimization of Cd3P2 slabs (with and without capping agents); an MP type k-point grid of 1 × 1 × 1 was used for the optimization of Cd3P2 slab with capping agents and ReC0A. The capping agents were modeled as formic acid. During structure optimization, the Cd3P2 unit cell's atom position, cell shape and cell volume were allowed to relax; for Cd3P2 slabs, Cd3P2 slabs with capping agents, and Cd3P2 slabs with capping agent & ReC0A only atom positions were allowed to relax. However, atoms in the bottom half of Cd3P2 slabs were frozen at their bulk positions. Both ReC0A binding geometries were optimized with the same unit cell and number of molecules. For the single carboxylic acid case, the ReC0A was optimized from a monodentate structure. In the case of dicarboxylic acid binding, the complex was optimized from a bidentate structure. The stable (100) facet of Cd3P2 was selected as the exposed surface to model Cd3P2 quantum dots. For all Density of State (DOS) calculation, the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional 34 was applied. Considering the size of the model, a MP type k-point grid of 1 × 1 × 1 was applied.

S16. Quantum Dot Area Calculation
In experiments, the QD diameter is 3.1 nm and the QD concentration is 25 μM. From the simulation model, we estimated for both ReC0A's carboxyl functional group to be bound on the QD surface, the minimum surface area require is 1.06 nm 2 per ReC0A molecule. The experimental QD's surface area is 30.19 nm 2 , thus about 28.5 (31.19 nm 2 /1.06 nm 2 ) ReC0A molecules can be adsorbed on the QD through both carboxyl groups. If the number of ReC0A molecules is more than 28.5 per QD, some of them can only have single carboxyl bound to the surface, with the second carboxyl group protonated and pointed away from QD. For a solution with 25 μM QD, the critical concentration for ReC0A to change its surface binding mode from double carboxylate to single carboxylate will be 0.71 (28.48*25 μM* 1/1000 mM/μM) mM, which is within the same range of experimental concentrations to see the disappearance of the 1700 cm -1 peak. The maximum effective concentration of surface protons should be 2.84 mM (4 times site number compared to ReC0A binding site)